Those familiar with the characteristics of HID lamps are aware of the difficulty encountered when attempting to restart a lamp while it is hot after deionization due to a temporary power line dip or interruption. Without special circuit arrangements, such lamps cannot be restarted, or are at least quite difficult to restart, until the hot, high pressure plasma gases cool down.
Then, when the lamp is cool, the fill gas is ionized by a low voltage and high current, but generates very little light output. The elements gradually heat up and re-evaporate into plasma components which then go into electrical discharge, forcing the lamp to produce the normal amount of light. While the cool-down and restart sequences are occurring there is no light output produced by the lamp. The length of time can be significant: a high pressure sodium (HPS) lamp typically requires 2-3 minutes to restart in a hot fixture and 1-2 minutes more to warm up. Mercury lamps typically require 4-7 minutes and metal halide 4-15 minutes.
A number of circuits have been developed for the purpose of starting such lamps while they are hot to avoid the absence of light discussed above, and other circuits have been developed which simply wait a predetermined time so that restarting can be accomplished after the lamp has completely cooled. Examples of circuits which attempt to deal with the hot restart problem are found in the following U.S. Pat. Nos.
3,476,977 Hallay PA1 3,732,460 Wattenbach PA1 3,749,968 Jones et al PA1 3,944,876 Helmuth PA1 3,991,344 Tabor PA1 4,048,539 Walker et al PA1 4,103,209 Elms PA1 4,240,009 Paul PA1 4,275,337 Knoble PA1 4,320,325 Anderson PA1 4,339,692 Lasecki et al PA1 4,339,695 Siglock PA1 4,356,433 Linden PA1 4,378,514 Collins PA1 4,382,210 Buhrer PA1 4,399,392 Buhrer PA1 4,403,173 Mayer PA1 4,437,042 Morais et al PA1 4,438,369 Hicks et al PA1 4,455,510 Lesko PA1 4,461,982 Faehnrich PA1 4,464,607 Peil et al PA1 4,495,446 Brown et al PA1 4,506,195 Elms
Some circuits which have reached the market are unsuitable because they simply do not work or are not reliable, while others are unsuitable because they are unsafe. These devices apply very high voltage pulses having long base times to the wires, the sockets, the screw-shell base and the lamp structure until the hot lamp restarts, whereupon the high voltage pulses are turned off. This is a potentially dangerous situation since the presence of the high voltage is not visually apparent. Some circuits also continue to apply high voltage pulses after a lamp has failed, a condition which can result in destruction of the lamp and socket as well as presenting a danger to service personnel. Those circuits which supply pulses continuously to a lamp in an effort to hot-restart the lamp can cause the lamp to go into what is called a glow or "latched up" mode. Once the lamp has entered this mode it will never restart. Some such devices have been found to cause internal lamp arcing rather than the anticipated hot restarting, resulting in the destruction of new lamps. It is thus apparent that improvements in this area are needed.